1. Field of the Invention
This invention relates to a method for measuring oxygen partial pressures and an oxygen partial pressure sensor and, more particularly, to detecting oxygen partial pressures by measuring the change in electronic conductance in an electrolyte that is sandwiched between an oxygen permeable electrode and an oxygen impermeable electrode.
2. Background of the Invention
The need to reduce unwanted emissions and improve fuel economy has prompted research into new oxygen sensors for internal combustion engines. The use of an exhaust-gas sensor in a feedback control arrangement allows for the regulation of the intake air-to-fuel ratio and has gained widespread use in the automobile industry.
Two types of sensors are currently utilized. The first type of sensor utilizes an oxygen pump concept and is disclosed in U.S. Pat. No. 5,010,762, and in U.S. Statutory Invention Registration No. H427. In an oxygen pump arrangement, molecular oxygen from an exhaust stream first is reduced to oxygen anions at a negatively charged, oxygen ion-porous electrode. These anions pass through the porous electrode toward another porous electrode that is positively charged. Upon reaching this second electrode, an inverse electrochemical reaction occurs whereby the oxygen anions are released as O.sub.2 as they pass through the second electrode. Obviously, the utilization of two porous electrodes is crucial to any basic oxygen-pump configuration.
Another type of sensor involves comparing the subject gas with a reference gas for oxygen content. For example, U.S. Pat. No. 5,017,499 utilizes packed columns to subject fluorine gas to a halogen-lacking material to form solid fluoride and either molecular oxygen or carbon dioxide. The concentrations of the oxygen-containing compounds are then compared with a reference gas, such as air.
Another method which uses a reference gas for detecting oxygen partial pressures comprises measuring the open-circuit potential, between two porous electrodes, that is generated by the difference in the exhaust-gas oxygen partial pressure relative to a reference gas. In these arrangements, the reference gas is brought into contact with one surface of a solid electrolyte while an opposite surface of the electrolyte contacts the gas to be analyzed. The partial pressure of oxygen (p.sub.O2) in the gas can be determined by measuring the current flowing between the electrodes situated on opposite sides of the electrolyte layer, Such a design is disclosed in U.S. Pat. No. 4,980,042. These potentiometric devices are often referred to as .lambda.-sensors. The .lambda.-sensor is conceptually simple and is based upon the principle that by sensing the oxygen content in an exhaust-gas stream, it is possible to approximate the engine equivalence ratio. The engine equivalence ratio is defined as follows: ##EQU1##
The electrode of the .lambda.-sensor that faces the exhaust gases usually contains platinum, as platinum yields a nearly reversible oxygen electrode necessary for the open-circuit potential measurement of the .lambda.-sensor. Also, platinum catalyzes the oxidation of gaseous species thereby lowering the oxygen partial pressure at the sensor surface relative to its exhaust-gas value. While this provides a sharp, and therefore easily distinguishable change in the .lambda.-sensor response when the fuel mixture changes from rich to lean or from lean to rich, the disadvantage of such a catalyzed electrode surface is that the oxygen partial pressure corresponding to the .lambda.-sensor output is lower than that of the actual exhaust-gas since the platinum electrode facilitates reaction of some of the oxygen with other exhaust gas constituents. Hence, if control of equivalence ratios away from stoichiometry is desired, the utility of the catalyzed .lambda.-sensor is limited to sensing a deviation from the theoretical efficiency value .lambda. without measuring the degree of such a deviation.
Generally, oxygen sensors currently used to control the intake air-to-fuel ratio of an automobile perform well near the stoichiometric air to fuel ratio but are not able to provide precise control during fuel-lean or fuel-rich operations.
Also, oxygen sensors currently in use in automobiles essentially are concentration cells with the voltage varying with the log of oxygen partial pressure. These sensors have relatively low sensitivity at elevated temperatures of 100.degree. C. to 1000.degree. C. Conversely, sensors that are useful at higher temperatures and particularly those for automobiles have relatively low sensitivity as to the change in the signal per change in oxygen partial pressure.
Therefore, despite a myriad of types of oxygen partial pressure sensors, a need still exists for a sensor and a method to detect oxygen partial pressures with relatively high sensitivity in widely varying fuel-rich and fuel-lean mixtures and at temperatures of 100.degree. C. to 1000.degree. C. as well as at temperatures above 1000.degree. C.